Vehicle braking force control system

Abstract

A braking force control system controls the brake fluid pressure applied to the wheel during braking when each of the wheels of the vehicle tends to lock so that the slip ratio of the wheel converges on a target slip ratio. The brake fluid pressure is once fixed at the beginning of the braking force control, and when the slip ratio falls below the target slip ratio by fixing the brake fluid pressure, the brake fluid pressure is increased at a first rate, and when the slip ratio does not increase to a predetermined slip ratio in a predetermined time after the slip ratio falls below the target slip ratio or when the rate of increase of the slip ratio after the slip ratio falls below the target slip ratio is lower than a predetermined rate, the brake fluid pressure is increased at a second rate which is higher than the first rate.

We claim: 
     
       1. A braking force control system for a vehicle comprising a slip ratio detecting means for detecting an actual slip ratio of each wheel of the vehicle, a lock detecting means for detecting that a respective wheel tends to lock and a braking force changing means which changes a braking force applied to each wheel during braking when the lock detecting means detects that the respective wheel tends to lock so that the actual slip ratio of the respective wheel converges on a target slip ratio, wherein said braking force changing means fixes the braking force at a predetermined value when the lock detecting means detects that the respective wheel tends to lock, increases the braking force at a first rate when the actual slip ratio falls below the target slip ratio as a result of fixing the braking force, and increases the braking force at a second rate which is higher than the first rate when the actual slip ratio does not increase to a predetermined slip ratio within a predetermined time after the actual slip ratio falls below the target slip ratio. 
     
     
       2. A braking force control system as defined in claim 1 in which said target slip ratio is defined as a range having upper and lower limits, and said braking force changing means increases the braking force at the first rate when the actual slip ratio falls below the lower limit of the target slip ratio range. 
     
     
       3. A braking force control system as defined in claim 2 in which said predetermined slip ratio is lower than the lower limit of the target slip ratio range by a predetermined amount. 
     
     
       4. A braking force control system as defined in claim 2 in which said predetermined slip ratio is equal to the lower limit of the target slip ratio range. 
     
     
       5. A braking force control system for a vehicle comprising a slip ratio detecting means for detecting an actual slip ratio of each wheel of the vehicle, a lock detecting means for detecting that a respective wheel tends to lock and a braking force changing means which changes the braking force applied to the respective wheel during braking when the lock detecting means detects that the respective wheel tends to lock so that the actual slip ratio of the respective wheel converges on a target slip ratio, wherein said braking force changing means fixes the braking force at a predetermined value when the lock detecting means detects that the respective wheel tends to lock, increases the braking force at a first rate when the actual slip ratio falls below the target slip ratio as a result of fixing the braking force, and increases the braking force at a second rate which is higher than the first rate when the rate of increase of the actual slip ratio after the actual slip ratio falls below the target slip ratio is lower than a predetermined rate. 
     
     
       6. A braking force control system as defined in claim 5 in which said target slip ratio is defined as a range having upper and lower limits, and said braking force changing means increases the braking force at the first rate when the actual slip ratio falls below the lower limit of the target slip ratio range.